Preheat your oven to 400°F.

Scrub the potatoes under running water and pat them dry. Pierce each potato several times with a fork or the tip of a knife. Place them directly on the oven rack and bake for about 45 minutes, or until the skins are crisp and the insides are tender.

While the potatoes bake, heat a large skillet or cast-iron pan over medium-high heat. Add the ground beef and cook, breaking it into crumbles, until browned. Season with 1/4 teaspoon of salt, then transfer the beef to a bowl and set aside.

In the same skillet, melt 1 tablespoon of the butter over medium-low heat. Add the diced onion and cook, stirring often, until softened and starting to caramelize, about 8 minutes. Stir in the minced garlic and cook for 1 minute more. Remove from heat and add the onion-garlic mixture to the cooked beef.

In the same skillet, melt the remaining 3 tablespoons of butter over medium heat. Whisk in the flour and cook for 1 minute to eliminate the raw flour taste. Gradually pour in the beef broth and milk, whisking constantly. Stir in the Worcestershire sauce, Dijon mustard, 1/4 teaspoon salt, and black pepper. Add the shredded cheddar cheese and continue stirring until the sauce thickens, about 3 minutes. Return the beef and onion mixture to the skillet, combining well.

When the potatoes are fully cooked, remove them from the oven and let cool slightly. Slice each potato open lengthwise, fluff the interior with a fork, and generously spoon the cheesy beef mixture on top. Sprinkle with chopped parsley and serve immediately.